Liquid crystal display devices have become rapidly popular in place of cathode ray tube (CRT) based display devices in recent years. The liquid crystal display devices have been in widespread use in liquid crystal televisions, monitors, mobile phones, and the like, which take advantage of, e.g., energy saving, thin, and lightweight features of the liquid crystal display devices. One way to further take advantage of such features is to improve an illumination device (i.e., a so-called backlight) which is provided behind the liquid crystal display device.
The illumination devices are roughly classified into a side light type (also referred to as an edge light type) and a direct type. The side light type is configured such that a light guide plate is provided behind a liquid crystal display panel and that a light source is provided at a lateral edge of the light guide plate. Light emitted from the light source is reflected by the light guide plate, so as to irradiate the liquid crystal display panel indirectly and uniformly. With this configuration, it is possible to realize an illumination device which has a reduced thickness and excellent luminance uniformity, although its luminance is low. For this reason, the side light type illumination device is mainly used in medium- to small-size liquid crystal displays such as a mobile phone and a laptop personal computer.
One example of the side light type illumination device is the one disclosed in Patent Literature 1. Patent Literature 1 discloses a surface-emitting device in which a reflecting surface of a light guide plate is provided with a plurality of dots for the purpose of allowing for uniform light emission from a light-emitting surface. In this surface-emitting device, light is not transmitted to a corner section of the reflecting surface due to directivity of a light source, and thereby the corner section of the reflecting surface is darkened. In order to deal with this, the corner section has a higher dot-density compared with other sections.
The direct type illumination device is provided with a plurality of light sources aligned behind a liquid crystal display panel, so as to directly irradiate the liquid crystal display panel. This makes it easier to obtain a high luminance even with a large screen. On this account, the direct type illumination device is mainly employed in a large liquid crystal display of 20 inches or more. However, a currently-available direct type illumination device has a thickness of as much as approximately 20 mm to approximately 40 mm, and this becomes an obstacle to a further reduction in a thickness of the display.
The further reduction in the thickness of the large liquid crystal display can be achieved by shortening a distance between the light source and the liquid crystal display panel. In this case, however, it is impossible for the illumination device to achieve luminance uniformity unless the number of light sources is increased. However, increasing the number of light sources increases a cost. In view of this, there is a need for developing an illumination device which is thin and has excellent luminance uniformity, without increasing the number of light sources.
Conventionally, in order to solve these problems, such an attempt has been conducted that a plurality of side light type illumination devices are aligned and thereby the thickness of the large liquid crystal display is reduced.
For example, Patent Literature 2 proposes a planar light source device that can secure a wide light-emitting area with a compact structure and therefore can be suitably used in a large liquid crystal display. This planar light source device has a tandem structure in which board-shaped light guide blocks are aligned tandemly and each of the light guide blocks is provided with a first light source for supplying each of the light guide blocks with first light.
An illumination device configured, as described above, such that a plurality of light-emitting units each of which is made by a combination of a light source and a light guide plate are aligned is called a tandem type illumination device.
Citation List
Patent Literature 1
Japanese Patent Application Publication, Tokukai, No. 2003-43266 (Publication Date: Feb. 13, 2003)
Patent Literature 2
Japanese Patent Application Publication, Tokukaihei, No. 11-288611 (Publication Date: Oct. 19, 1999)
Patent Literature 3
Japanese Patent Application Publication, Tokukai, No. 2006-302687 (Publication Date: Nov. 2, 2006)